Background: Mastitis is a common disease of small ruminants causing major economic losses due to reduced yield and poor quality of milk worldwide. The problem is even more relevant in the rural areas of the Mediterranean region, where almost two thirds of the global sheep and a quarter of the global goat milk are produced. The current investigation aimed to gain insight into the genetic diversity, antimicrobial resistance profiles and virulence associated factors of Staphylococcus (S.) aureus isolated from animals with clinical mastitis in dairy farms in Sardinia, Italy.

Methods: Thirty-four Staphylococcus aureus isolates collected from clinical mastitis cases from sheep and goat from 26 dairy farms were characterized using whole genome sequencing and antibiotic resistance testing.

Results: All isolates proved to be phenotypically methicillin sensitive S. aureus (MSSA). Only few isolates showed resistance against tetracycline (12.5%) and erythromycin (3.12%). The isolates were assigned to seven different sequence types: ST133 (n = 15), ST700 (n = 9) were the main two sequence types. Resistance genes blaZ, dfrG and mecA were found in one isolate, each (3.12%). All tetracycline resistant isolates harbored either tetM or tetK.

Conclusion: ST133 had the highest potential to cause disease and was found frequently in sheep mastitis cases. Seven different sequence types were identified among a limited number of isolates. The circulation of some clusters in different regions shows the persistence of cluster-associated infection. Although all isolates were from clinical mastitis cases, they have showed almost no resistances to the tested antimicrobials. This indicates the highly developed hygienic measures in the surveyed farms, and the routine antimicrobial resistance testing in combination with prudent use of antimicrobials to avoid the emergence of resistance. This is in line with the current agricultural regulations, i.e., implementation of EU legislation, increases the farmers´ income through avoiding losses due to reduced milk yield and decreases the veterinary costs. Most importantly, the burden of disease is reduced and animal welfare improved.